Recently, liquid crystal display devices have been widely used as display devices of portable devices such as smartphones and tablet computers or display devices of vehicle-mounted devices. In general, a liquid crystal display device comprises a liquid crystal display panel, and an illuminating device (backlight device) which is opposed to the back surface of the liquid crystal display panel and illuminates the liquid crystal display panel.
The illuminating device comprises a light guide plate having a light emission surface which is opposed to the liquid crystal display panel, an optical sheet which overlaps the light guide plate, a reflection sheet which is opposed to the back surface of the light guide plate, and a light source which emits light to the light incident surface (end surface) of the light guide plate. The light which has entered the light guide plate is repeatedly reflected inside the light guide plate and is emitted from the entire light emission surface toward the liquid crystal display panel.
Recently, in the display device, a method of increasing the contrast of a display image by individually controlling the light amounts of a plurality of light sources, that is, the so-called local dimming control has been proposed. However, in the liquid crystal display device, the light guide plate cannot sufficiently concentrate the light entering from the end surface thereof, and the light is emitted from the entire backlight, that is, from the entire light guide plate. Thus partial illumination is technically difficult. Therefore, it is difficult to perform the local dimming control in the above-described liquid crystal display device.